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Category name:
Ni Ion - Nitrate

Justifications and discussions

Category definition:
Toxicity is attributed to the nickel ion and the role of the counter-ion in the resulting toxicity is insignificant
Category rationale:
In environmental toxicity tests, nickel toxicity is assessed by exposing the organism(s) to increasing doses of a soluble nickel salt. The nickel salt dissolves in the test medium, causing the cationic nickel ion to dissassociate from the anionic counter-ion. As a result, the test organism(s) will simultaneously be exposed to the nickel ion, the counter-ion, and some proportion of the undissociated complex, and the observed effect will be the combination of the effects exerted by all of these factions. Within these tests the concentrations of the nickel ions are sufficient to cause toxicity, while the concentrations of the corresponding "counter-ions" are not expected to elicit a toxic response. Therefore, the identity of the corresponding counter-ion is null, and the toxicity may be read-across on the basis that the nickel ion is responsible for the observed toxicity.

The attached document assesses under what conditions, if any, the counter-ion may contribute to the observed toxic effects of nickel in the aquatic, terrestrial or sediment compartments. This document begins with a determination of the solubility and dissociation constants for the nickel substances. From here, each environmental compartment (aquatic, terrestrial and sediment) is examined by summarizing the relevant toxicity data and risk assessment thresholds for nickel, the background exposure concentrations of nickel, the toxicity data and risk assessment thresholds associated with anions, and background (exposure concentrations) of anions in a given compartment. A comparison between nickel and anions is made for each compartment that incorporates the toxicity data, environmental reference values, and exposure data. This analysis is ultimately used to derive a proposed “screening tool” which can be used to assess the relative toxicity contribution that stems from the nickel ion and the counter ion individually.

Following the summation of the attached document, these rules are applicable to environmental toxicity studies that occur within water, sediment and soil and are relevant with no dependence upon test duration (acute/chronic) nor taxa.